This invention relates generally to the art of the microwave heating by high frequency electromagnetic radiation, or microwave energy. The invention has broad utility in microwave heating of a multitude of materials, including food products.
Microwave heating offers certain advantages over other heating techniques. For example, it heats fast, efficiently and is penetrating, making it possible to heat products, such as foods, rapidly throughout. This is particularly valuable in the food art for consumer cooking or reheating of previously prepared foods. It also may be utilized in other arts wherein fast heating and drying of products are desired; for example, in the construction industry for rapid drying of concrete, lumber drying, textiles, rubber, ceramics or the like.
In the past few years, microwave heating has enjoyed considerable popularity with the public, primarily due to convenience factors stemming from the rapid heating rates which can be achieved.
However, while microwave heating is currently enjoying considerable popularity, it also has some deficiencies which would make it even more desirable if the deficiencies could be overcome. Amongst these deficiencies are lack of uniformity of heating throughout an entire product, the inability to successfully crisp or brown surfaces of food products such as pizza, pie crust, breads, meat pies, crispy snack foods, biscuits, french fries, and the like. Indeed, one common occurrence when temperatures are elevated sufficiently high to crisp or brown will be over-cooking, scorching, charring or burning of portions of the product. This, of course, does not meet with public acceptability. Conversely, if microwave heating is accomplished at lower temperatures, below the temperatures needed for crisping or browning, often internal portions of the food product do not become totally cooked, and moisture is driven towards the outer surfaces of the product and remains there, giving an overall impression of sogginess. This is not desirable.
As is understood, by those skilled in the art of microwave heating, the ability or lack of ability of any given material to absorb microwave energy and convert that to heat energy is measured in terms of the lossy characteristics of the substance. Substances which will absorb microwave energy and convert it to heat energy are known as "lossy". On the other hand, substances which will not absorb microwave energy are said to be non-lossy or microwave transparent.
Particularly in the food art, one approach to solving the dilemma expressed above in order to provide browning and crisping of surfaces is to provide a heating vessel which has, at least on one surface of the vessel, a lossy heater. Such heaters are in reality materials highly susceptible to microwave absorption and heat conversion, and being very lossy, the result is that these surfaces heat to a substantially elevated temperature. Thus, portions of a food product which are in thermal contact with such surfaces will be heated to a substantially elevated temperature in comparison with the bulk of the food product, resulting in browning or crisping.
Examples of such ceramic heaters include ferrites, semiconductors, and the like. For an example of a cooking vessel employing a lossy ceramic heater, see Sumi, et al., U.S. Pat. No. 3,941,967, which teaches a microwave cooker of the casserole type. The vessel is permanent, non-disposable in nature, and employs a ferrite ceramic heating element. Examples of ferrite ceramic heating elements include nickle zinc ferrite, magnesium zinc ferrite, barium ferrite, and strontium ferrite.
While such ceramics have met with some success as heating elements for use with microwave energy, they also have considerable drawbacks. Ceramic heating elements are expensive; they add considerable bulk and weight to packaged products and do not readily lend themselves to employment with disposable non-permanent packaging materials; and, perhaps most importantly, ceramic heating elements may provide for uncontrolled (run away) heating to elevated temperatures. This often results in scorching, charring and burning.
Another example of a lossy heater often used is tin oxide, a semiconductor heater on a glass substrate. These are massive and have the same general deficiencies as the ceramic heaters.
Thus, in summary, while ceramic and semiconductor heaters certainly have their place in microwave technology, they also have considerable deficiencies for some uses. Among those deficiencies are expense, and a seeming inability to regulate and control maximum temperature achieved.
The invention relates to the development of an entirely new class of microwave heater materials. The materials suitable for use in this invention have a unique capability of initially being lossy, and after continued exposure to microwave energy, they reach a certain elevated maximum temperature, at which time, due to either chemical or physical phenomena or a combination of both, they become non-lossy and substantially microwave transparent. As a result, the temperature-time profile of microwave heating can be substantially predetermined; the maximum temperature achievable can be determined and predicted, and the microwave absorber can be "tailor made" for a particular heating job.
The materials which are usable will be explained in detail in the Description of the Invention which follows; however, they are referred to herein as "chemical susceptors". As used here, the term "susceptor" or "susceptor device" refers to a device for converting microwave energy into heat which in turn heats another article placed on or nearby the susceptor. To be efficient, the susceptor should heat more rapidly in the microwave field than the article to which its thermal energy is to be transferred. The term "chemical susceptor" as utilized herein means material which is initially lossy and which upon continued exposure to microwave energy, reaches a certain, ascertainable maximum temperature and thereafter becomes substantially non-lossy, or microwave transparent.
Accordingly, one object of this invention is to provide an entirely new class of microwave lossy materials which are initially lossy at ambient temperature and which eventually upon continued heating by exposure to microwave energy, become substantially non-lossy.
Another object of this invention is to provide an entirely new class of non-ceramic chemical susceptors usable for microwave heating of almost any product material, including foods.
Another object of this invention is to provide a new and unique method of providing a desired heating profile in a microwave field by manipulation of the formulation of a chemical susceptor.
Yet another object of this invention is to provide a disposable microwave heating package which employs the chemical susceptors of this invention.
A still further object of this invention is to provide a disposable microwave heating package which does not employ ceramic lossy absorbers, and which can be effectively used for selective dehydration of surfaces of a product to be treated to provide browning, crisping or the like.
Yet another object of this invention is to provide a microwave heating package which is inexpensive, flexible and disposable, and also which is particularly adapted for use as a carton for vending machine use.
A yet further object of this invention is to provide regulation of heat load for a packaged food product by formulating the product to increase or maximize the ability to heat rapidly.
The manner and method of accomplishing each of the above stated objects, as well as others, will be apparent from the description which follows.